Semiconductor microcavity detectors and light sources are essential components of emerging integrated photonic systems and micro-opto-electro-mechanical systems. The present document relates to a plurality of these semiconductor microcavity devices of both the optical energy emitting and optical energy receiving types. The vertical cavity surface emitting laser and the resonant cavity light emitting diode are two particular examples of these optical energy emitting devices and are generally referred to as microcavity light emitters in this document. The resonant cavity heterojunction phototransistor is an example of the microcavity devices which are generally referred to as microcavity detectors herein.
Vertical emitting microcavity light sources improve on many attributes of the commonly used semiconductor edge emitting lasers. Conventional edge emitting semiconductor lasers emit light in a direction parallel to the semiconductor substrate on which the lasers are formed. A vertical cavity surface emitting laser, however, has an optical cavity located perpendicular to the substrate and emits optical radiation in a direction perpendicular to the substrate. A few of the many advantages of vertical cavity surface emitting lasers include the capability of fabricating the devices at a much smaller size and that the light emitted from vertical emitting microcavity sources has a circular shape, as opposed to the oblong shape of the light from edge emitting lasers. These microcavity devices consist of a quantum well light absorbing or light gaining region, a region within an optical microcavity that is bounded by distributed Bragg reflector mirrors. Various combinations of monolithically integrated vertical cavity surface emitting lasers, resonant cavity light emitting diodes, resonant cavity PIN photodetectors, and resonant cavity heterojunction phototransistors have been fabricated. Because of the similarity of the device structures, it is in fact possible to construct several of the different device types during a single epitaxial growth sequence. To achieve this, individual devices positioned side-by-side or in a stacked configuration may be fabricated from multiple purpose vertical cavity surface emitting laser structures by subsequent selective etching, native Al-oxidation or ion implantation, and metallization steps. The devices may be interconnected functionally or functionally discrete and a functional interconnection if used may include both optical and electrical interconnection as in the present invention or any optical interconnection or any electrical interconnection. Both the vertical cavity surface emitting laser and resonant cavity light emitting diode are emerging in commercial applications. The vertical cavity surface emitting laser is more efficient than the resonant cavity light emitting diode and produces a stronger optical power output, but is generally more difficult and costly to manufacture than the resonant cavity light emitting diode.
An important requirement for optical data storage and optical communication systems, using either edge emitting lasers or microcavity light emitting devices is the dynamic stabilization of the device and its optical output power. Such stabilization is necessary to correct fluctuations due to age and changes in the environment such as from heat generated by the laser operation, or long-term drift of the laser properties. For example, conventional audio compact disk optical pick-up heads use a silicon PIN photodiode to monitor the optical power that escapes from a back facet of an edge-emitting laser diode in order to control the laser's output power level. Laser diodes used in fiber optic communication systems also use a photodetector to monitor the laser output power. For conventional edge-emitting lasers, the output power of the laser is monitored by a separate photodetector installed in the vicinity of the laser. A slightly leaky facet allows some of the emitted laser optical output power to impinge on the photodetector. The photodetector couples a signal that corresponds to the intensity of the emitted radiation to a feedback control circuit which adjusts the current driving the laser until the desired output power is achieved. In this manner, any drift of the laser intensity is detected and compensated for by adjusting the current applied to the laser.
A similar monitoring scheme for the automatic power control of vertical cavity surface emitting lasers is also known. In this approach, a PIN photodiode is grown on top of a top-emitting vertical cavity surface emitting laser in a single growth sequence. Except for the intrinsic absorbing layer and normally small free carrier losses, the PIN photodiode is transparent to the laser emission. However, a drawback of this approach is that the PIN photodiode Pinphotodiode produces a small signal that corresponds to the intensity of the laser output power plus the internally generated spontaneous emission. Consequently, the photocurrent generated by the PIN photodiode requires additional external circuitry to amplify the signal and this adds to the size and expense of a complete system. A second drawback is that a large fraction of the feedback signal is produced in response to spontaneous emission, the non-lasing light activity that occurs at all times, and this introduces error into the feedback signal. This error must be accounted and corrected for in the feedback circuit. Another drawback is that a PIN photodiode is most readily fabricated in a layer arrangement which is not easily compatible electrically with the layers needed in the relevant portion of a microcavity light emitting device- as is explained in detail subsequently herein.
In addition to the PIN photodiode monitoring of the laser output power, it is also known to fabricate a functionally independent transistor in the same layer structure used to accomplish a microcavity light emitting device. As is also explained in greater detail subsequently herein, however, there is believed to be a significant difference in both the structure and function of the power monitoring transistor arrangement of the present invention and the structure and function of functionally independent transistors. The present invention is moreover believed to involve a fundamental concept in the arrangement of any device for microcavity light emitting power control purposes which has not been appreciated heretofore.